In the production of disposable articles, hot melt adhesives are typically extruded at elevated temperatures (about 250.degree. F. to 350.degree. F.) directly onto a work piece, typically a polyethylene or polypropylene film, a nonwoven fabric, an absorbent material, a tissue, or a film which can then be bonded to another nonwoven fabric, absorbent material, tissue, or film using the hot melt adhesive. Application of the adhesive may be extruded by fine line, multi-dot, multi-line methods or spray techniques. The hot melt adhesive is used to laminate layers or attach elastic, particularly for waist or leg band closures for disposable diapers; elastic attachment also results in laminating.
Since in the assembly of disposable articles a variety of materials are bonded under a wide range of conditions, to optimize performance separate adhesives have evolved for use in the manufacture of disposable articles. This is particularly true in elastic attachment and in laminate construction. Construction requires an adhesive with controllable, relatively low viscosity, long open time, and sufficient bonding strength to maintain the mechanical integrity of the laminate. However, to bond elastic materials to substrates, a different adhesive which exhibits high creep resistance to ensure that the elastic, when under stress, does not move relative to the surfaces of the substrates or become partially or fully detached, is used.
The use of separate adhesives for laminate construction and for elastic attachment purposes, each having different formulas and properties, increases disposable article manufacturing complexity and can reduce productivity. Additionally, if multiple adhesives are required inventory and storage problems are increased. If the incorrect adhesive is used for elastic bonding it can lead to bond failure. Also, the applicator can become plugged. Such problems can lead to inferior products, lost production or both.
Accordingly a substantial need exists for a single adhesive having properties rendering the adhesive applicable to both lamination and elastic bonding applications. Such adhesives are known as multipurpose adhesives.
Construction adhesives are soft (tacky), have high peel strength, long open time and low cohesive strength. Elastic attachment adhesives are stiffer (not as tacky), high in cohesive strength, and shorter in open time, than construction adhesives. For multipurpose applications, in particular, elastic attachment, the balance of stiffness, cohesive strength, and open time is required for good performance.
Styrene-isoprene-styrene or "SIS" block copolymers are commonly used for elastic attachment in disposable products. SIS is chosen because, when compared to other block copolymers, for the same melt index and rubber content, SIS polymers provide a higher molecular weight and softer adhesive products. Adhesives used for elastic attachment need to contain high amounts of rubber and endblock resin to obtain adequate cohesion. For example, U.S. Pat. No. 5,149,741 to Alper discloses elastic attachment adhesives comprising 35 parts of SIS copolymer, in combination with 10 parts endblock resin.
As used herein, the "midblock" of the polymer refers to polymeric blocks which are substantially aliphatic. As will be discussed below, "midblock resin" refers to a tackifier which is compatible with the midblock of the polymer. "Endblock" of the polymer refers to polymeric blocks which are substantially aromatic. "Endblock resins", as will be discussed in detail below, are substantially aromatic and compatible with the endblock of the polymer.
At similar polymer content, melt index, and endblock resin level, a corresponding SBS-based adhesive is not as pressure sensitive as SIS due to high stiffness. The higher stiffness results from the lower entanglement molecular weight (higher entanglement density) of butadiene versus isoprene. U.S. Pat. No. 5,071,571 (Malcolm) discloses an adhesive for elastic attachment comprising low content of a very high molecular weight SBS copolymers.
Endblock resins are commonly used with SIS to improve cohesive strength in pressure sensitive adhesives. Endblock resins are not commonly used with SBS because it is difficult to ensure that the resin actually reaches the endblock due to the higher polarity of the butadiene midblock compared to isoprene midblock, which therefore tends to solubilize the endblock resin in the midblock to a large extent. In addition, endblock resins are not commonly used in SBS-based elastic attachment adhesives. U.S. Pat. No. 4,944,993 (Raykovitz) discloses low molecular weight, radial SBS polymers comprising a styrene content greater than 35% with endblock, and their use in construction and elastic attachment adhesives.
Endblock resins are effective only when they associate with the styrene portion of the block copolymer chain. Due to the closeness in solubility parameter of butadiene and styrene, the resin associates with the midblock or butadiene portion in low molecular weight SBS copolymers. However, in high molecular weight SBS copolymers, the longer butadiene chain does not favor this association to the same extent. This effect can be demonstrated by rheology curves, where the Tg of the butadiene block can be followed. For example, when 10 parts midblock resin are replaced by endblock resin, the midblock Tg should, in theory, decrease. The higher the midblock Tg after such a substitution, the more the endblock resin is in the midblock, instead of the endblock, of the rubber. By using a high molecular weight rubber at low levels, in combination with endblock resin, in accordance with the present invention, an adhesive with high cohesive strength can be attained due to the association of the endblock resin with the styrene block of the rubber. In addition, because lower amount of rubber is used, the stiffness is low and hence, the tack is high; for the same reason, open time is still high enough for multipurpose applications.
Therefore, unexpectedly, it has been found, in accordance with the present invention, that a multipurpose adhesive, for use in both elastic attachment and construction, can be prepared from a high molecular weight radial or linear SBS copolymer, without a high styrene content, and endblock resins. The adhesives of the present invention have both good cohesive strength (elastic attachment) and long open time and low stiffness, for good lamination bonds. The high molecular weight SBS copolymer allows use of a lower amount of the copolymer providing a softer, more pressure sensitive adhesive, with longer open time, when compared to use of a low molecular weight SBS copolymer. In addition, the presence of the endblock resin provides for a higher cohesion, at lower viscosity, when compared to adding more copolymer to the system to obtain the same increased level of cohesion.
The present invention is directed to hot melt adhesive compositions suitable for disposable constructions comprising an endblock resin and a high molecular weight styrene-butadiene block copolymer having a molecular weight such that at 25% in toluene, the viscosity is greater than 1000 cPs.